The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A communication device (e.g., a mobile telephone) may include a single antenna for transmitting and receiving signals, or multiple antennas for transmitting and receiving signals, such as in a multiple in multiple out (MIMO) arrangement. The MIMO arrangement may include full division duplex (FDD) operation.
A single antenna may communicate with a duplexer that allows the antenna to function as both a receiver antenna and a transmitter antenna. For example, the duplexer selectively connects the antenna to a receiver portion of the device and a transmitter portion of the device. Insufficient (e.g., limited) isolation between the receiver portion and the transmitter portion, at the duplexer, allows coupling to occur between adjacent antennas (e.g., between a transmit frequency band and an adjacent receive frequency band). Accordingly, a single transmitter can impair operation of one or more receivers in the device.
For example, transmitter carrier leakage may interfere with operation of a low-noise amplifier (LNA) of the receiver portion, and may cause distortion, saturation, cross modulation, compression, etc. The interference may further lead to stringent receiver linearity (e.g., linearization that prevents transmit noise from being filtered prior to a power amplifier in the receive frequency band). The transmitter may also cause additive interference with the receiver portion, such as added transmitter noise, residual intermodulation products (IMs), spurs, etc. in the receive frequency band. Consequently, the receiver portion may be desensitized and power consumption of various components of the transceiver portion may be increased.
FIGS. 1A, 1B, and 1C show an example system 100 including a single antenna 104 for transmitting and receiving signals. The antenna 104 communicates with a duplexer 108, which is configured to provide communication, via respective ports of the duplexer 108, between a receiver portion 112 and a transmitter portion 116. As shown in a single carrier implementation with no linearization in FIG. 1B, a transmitter signal 120 generates transmitter IMs 124 and transmitter noise 128 (which is filtered by a zero intermediate frequency, or ZIF, reconstruction filter) in a transmit frequency band 132. However, minimal interference with a desired receive signal 136 occurs in a receive frequency band 140.
Conversely, in a multi-carrier implementation as shown in FIG. 1C, transmitter signals 144 and 148 generate transmitter IMs 152 and transmitter noise leakage 156 in both a transmit frequency band 160 and a receive frequency band 164, causing interference with desired receive signals 168 and 172.
FIG. 2 shows another example system 200. The system 200 corresponds to a 2×2 MIMO implementation including a first antenna 204 in communication with a first receiver portion 208 and a first transmitter portion 212 via respective ports of a first duplexer 216, and a second antenna 220 in communication with a second receiver portion 224 and a second transmitter portion 228 via respective ports of a second duplexer 232. Limited isolation (as indicated by the arrows) is present between the first antenna 204 and the second antenna 220). Nonetheless (e.g., by virtue of the distance between the first antenna 204 and the second antenna), this limited isolation may not prevent a transmitter (e.g., the first transmitter portion 212) from desensitizing a receiver (e.g., the second receiver portion 224).
Coupling between antennas (and corresponding transmitter and receiver portions) becomes more complicated in a 4×4 MIMO implementation, as shown in another example system 300 in FIG. 3. The system 300 includes a first antenna 304 in communication with a first receiver portion 308 and a first transmitter portion 312 via respective ports of a first duplexer 316, and a second antenna 320 in communication with a second receiver portion 324 and a second transmitter portion 328 via respective ports of a second duplexer 332. The system 300 further includes a third antenna 336 in communication with a third receiver portion 340 and a third transmitter portion 344 via respective ports of a third duplexer 348, and a fourth antenna 352 in communication with a fourth receiver portion 356 and a fourth transmitter portion 360 via respective ports of a fourth duplexer 364. Limited isolation (as indicated by the arrows) is present between various antennas and corresponding receiver and transmitter portions.
Various systems and methods may be used to isolate the receiver portions from the transmitter portions as shown in FIGS. 1-3. In one example, a passive ceramic or air cavity duplexer having a high transmit antenna attenuation and high transmitter-receiver isolation is used to prevent excessive leakage to the receiver. In another example, an active canceller implements a mixer to down convert an output of a transmitter power amplifier to a lower frequency. An output of the mixer is digitized with an analog to digital converter (ADC). An output of the ADC is equalized to match a delay, amplitude, and phase of the leakage via the duplexer and corresponding antenna. A digital signal processing system extracts a correlation between the transmitter leakage and the digitized power amplifier output to determine equalizer coefficients. A second path may be used to transmit the carrier leakage at the receiver LNA to prevent distortion and/or saturation. In still another example, radio frequency (RF) domain cancellation may be used.